The present invention is directed toward a temperature variable attenuator and more particularly toward an absorptive-type temperature variable microwave attenuator wherein the attenuation thereof changes at a controlled rate with changes in temperature while the impedance remains substantially constant.
Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter case is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications.
The important parameters of an absorptive attenuator are its accuracy as a function of frequency, its return loss and its stability over time and temperature. It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. Much time, effort and expense has gone into the components of such systems in an effort to stabilize them over various temperature ranges. This has greatly increased the cost of microwave systems that must be exposed to wide temperature ranges.
It is common today to find thermistors used in many types of electronic circuits. They are often employed as temperature compensating elements in analog circuits and as detectors in temperature probes. Most thermistor applications are at frequencies of a few hundred megahertz or below. To Applicant's knowledge, no one has ever considered utilizing the attributes of a thermistor in a microwave attenuator circuit that is usable up to 6 GHz or more.